Project 1: Defined Th1 responses causing intestinal inflammation such as that occurring in SJL/J mice with TNBS-colitis offer an excellent opportunity to identify genetic loci (and ultimately defined genes) that are responsible for the colitis. This approach is facilitated by the fact that there is enormous mouse strain variability in susceptibility to TNBS-colitis: SJL/J mice and C57BL/10 mice are highly susceptible whereas C25BL/6 mice are highly resistant. Accordingly, we conducted an extensive genome-wide linkage analysis in F2 progeny of SJL/J and C57BL/6 mice by identifying susceptible and resistant progeny and then correlating that with multiple chromosomal markers associated with the two strains. This analysis led to the identification of two susceptibility loci, one on chromosome 9 and one on chromosome 11 called TNBS1 and TNBS2 respectively. Furthermore, while TNBS was strongly associated with disease in male mice, it was relatively weakly associated with disease in females. Conversely, while TNBS was strongly associated with disease in female mice, it was not associated with disease in male mice. However, further analysis of resistant male mice negative for TNBS1 revealed an association with TNBS2; thus, both male and female mice have a susceptibility locus on chromosome 11. Recognizing that locus on chromosome 11 contains the IL-12 gene, we conducted futher studies to identify IL-12 response abnormalities that might also map to this locus. Accordingly, we administered IP LPS to SJL/J and C57BL/6 mice and showed that whereas IL-12 p40 production induced by the LPS was equivalent in the two strains, the amount of IL-12 p70 was vastly greater in the SJL/J strain than in the C57BL/6 strain. We then conducted further genome-analysis to map this difference in responses and found that the ability to mount a high IL-12 p70 response mapped to the SJL/J chromosome 11 allele. Thus, these data were strongly suggestive that a genetic abnormality involving the IL-12 gene is partly responsible for susceptibility to TNBS-colitis. This conclusion was also supported by the fact that male C57BL/10 mice whose genome is 99% identical to that of C57BL/6 mice and differs largely at the chromosome 11 susceptibility region, is also susceptible to TNBS-colitis and mounts a high LPS-induced IL-12 p70 response. Finally, it is important to note that a human chromosome region on human chromosome 5 (5q33-34) syntenic to the TNBS2 on chromosome 11 has been linked to human Crohn?s disease: on this basis, TNBS2 may have relevance to the human disease.